(1) Field of the Invention
The present invention relates to a webbing useful for safety belts. Particularly, the present invention relates to a webbing having an excellent impact energy absorption and a superior impact strength and useful for safety belts for cars and aircraft.
(2) Description of the Related Art
It is known that webbing to be used for safety belts for cars and aircraft is required to have a satisfactory absorption or mitigation of the impact force applied to a human body upon collision, and a satisfactory light weight, durability, color, pattern, design, and shape.
It is also known, however, that it is difficult to produce a webbing for safety belts having such a satisfactory impact force-absorption or mitigation property.
Several attempts to enhance the impact force-absorption or the webbing have been disclosed. For example, Japanese Examined Patent Publication (Kokoku) No. 53-1874 discloses a dynamic energy absorption belt in which two different types of yarns are used as warps and at least one type of the warp yarns is arranged in such a manner that the crimp percentage of the warp yarns increases with an increase in the distance from each side edge of the belt.
Japanese Examined Patent Publication (Kokoku) No. 53-2981 discloses a webbing for safety belts, having warps comprising high elongation filament yarns and low elongation filament yarns which are distributed among the high elongation filament yarns, and each of which low elongation filament yarns consists of first high tensile strength, low elongation filaments and second low tensile strength, low elongation filaments, the first high tensile strength, low elongation filaments being incorporated in a looped state with the second low tensile strength, low elongation filaments.
Japanese Examined Patent Publication (Kokoku) No. 54-19511 discloses a dynamic energy-absorption belt characterized in that two types of yarns having a different elongation from each other are used as warps, and that in the warps, at least one type of yarns, which are broken before the belt is ultimately broken, consists of at least two types of yarns having a different crimp percentage from each other.
Japanese Examined Patent Publication (Kokoku) No. 55-11053 discloses an energy-absorbing belt characterized in that two or more types of yarns consisting of the same type of polymer and having a different ultimate (breaking) elongation and initial elastic modulus, the ultimate elongation decreasing with an increase in the initial elastic modulus and the ultimate elongation increasing with a decrease in the initial elastic modulus, are used as warps, and in the weave structure of the belt, the crimp percentage of the warps is adjusted so that the crimp percentage decreases with a decrease in the ultimate elongation and increases with an increase in the ultimate elongation, and that the weave structure contains pores in a total volume of 40% or less. Also, Japanese Publication No. 55-11053 discloses a process for producing the energy-absorbing belt characterized in that a woven belt is produced from two or more different types of yarns capable of thermally shrinking, which yarns have been imparted with different crimp percentages, and the resultant woven belt is heat treated under tension.
The above-mentioned known dynamic energy-absorbing belts are intended to enhance the impact energy absorption property thereof by utilizing warps consisting of two or more types of yarns having a different tensile strength, elongation, and/or crimp percentage from each other.
According to the results of tests effected by the inventors of the present invention for the above mentioned types of energy-absorbing belts, it was concluded that the energy absorption by the above-mentioned types of belts is carried out stepwise, not continuously and smoothly.
It is true that the above-mentioned types of belts effectively decrease a sudden shock received by a human body, but the two or more different types of warps separately absorb the impact energy stepwise. Therefore, after one type of warps absorbs a portion of the impact energy, and before another type of warps absorbs another portion of the impact energy, the human body is exposed to an undesirable shock.
In order to eliminate the above-mentioned problem, Japanese Unexamined Patent Publication (Kokai) No. 59-179842 disclosed a webbing for safety belts comprising a woven belt which is characterized in that the warps consist of polyester filament yarn having an intrinsic viscosity of 0.7 or more; a birefringence of 0.03 to 0.13; an initial Young's modulus of 20 g/denier or more; an ultimate elongation of 80 to 200%; an elongation value of 5% or less at a point A in the tensile stress-strain curve of the yarns, which point A denotes an intersecting point of an extension line of a steeply sloped portion of the curve appearing in the initial stage of elongation of the yarns with an extension line of a substantially horizontal or slightly sloped portion of the curve appearing at the middle stage of elongation of the yarns; an elongation value of 30 to 60% at a point B in the curve, which point B denotes an intersecting point of the substantially horizontal or slightly sloped portion of the curve appearing in the middle stage of elongation of the yarns with a gently sloped portion of the curve appearing in the final stage of elongation of the yarns; and a ratio of the tensile strength of the yarns at a point C in the curve, which point C denotes a final end point of the curve, to that at the point A, of 1.5 or more.
This type of webbing exhibits a satisfactory impact energy absorption. It was found, however, that this type of webbing sometimes has an unsatisfactory tensile strength. Also, it was found that the polyester filament yarns having a birefringence of 0.03 to 0.13, an ultimate elongation of 80% or more, an elongation of 5% or less at the point A, an elongation of 30 to 60% at the point B, and a ratio (C/A) or 1.5 or more, usually exhibit a very poor tensile strength. Therefore, the resultant safety belt has an unsatisfactory tensile strength and is sometimes broken upon collision. That is, when a large load of 800 to 1000 kg is abruptly applied, the safety belt is suddenly prolonged to a large elongation, and is then broken.
Under the above-mentioned circumstances, a strong demand has arisen for a new type of webbing for safety belts having warps consisting of a single type of yarn and having a satisfactory impact energy absorption and an excellent resistance against breakage by impact stretching.